1. Field of the Invention
The present invention relates to a standard cell useful in the fabrication of a semiconductor integrated circuit on, for example, a silicon-on-insulator (SOI) substrate.
2. Description of the Related Art
Standard cells are widely used in the design of semiconductor integrated circuits. A conventional standard two-input NAND cell and a conventional two-input exclusive OR (XOR) cell will be described below. Descriptions of conventional two-input NAND cells can also be found in Japanese Patent Application Publication No. 2001-94054 and the corresponding U.S. Pat. No. 6,410,972 to Sei et al.
A standard two-input NAND cell is shown as a circuit diagram in FIG. 1 and a circuit symbol in FIG. 2. The two-input NAND cell performs a NOT-AND logic operation on two input signals a, b to generate an output signal y. The constituent elements of the cell are a p-channel metal-oxide-semiconductor (PMOS) transistor 1 and an n-channel metal-oxide-semiconductor (NMOS) transistor 3 connected in series to a power supply (vdd) rail, a PMOS transistor 2 connected in parallel with PMOS transistor 1 between the vdd rail and the drain electrodes of PMOS transistor 1 and NMOS transistor 3, and an NMOS transistor 4 connected in series with NMOS transistor 3 between the source electrode of NMOS transistor 3 and a ground (gnd) rail. Input signal a is applied to the gate electrodes of transistors 1 and 3, input signal b is applied to the gate electrodes of transistors 2 and 4, and the output signal y is taken from drains of transistors 1, 2, and 3. The output signal y goes to the low logic level when both inputs a, b are at the high logic level, because both NMOS transistors 3, 4 are turned on and both PMOS transistors 1, 2 are turned off. When at least one of the inputs a, b is low, the output signal y goes to the high logic level, because at least one of the NMOS transistors 3, 4 is turned off and one of the PMOS transistors 1, 2 is turned on.
The conventional two-input XOR cell is shown as a circuit diagram in FIGS. 3 and 4 and a circuit symbol in FIG. 5. The two-input XOR cell performs an exclusive logical OR operation on two input signals a, b to generate an output signal y. The constituent elements of the cell are three inverters 11-12, 12-22, 16-26, an analog switch 13-23, and a tri-state inverter 14-25. Inverter 11-12, which inverts input signal a and outputs the inverted signal /a, comprises a PMOS transistor 11 and an NMOS transistor 21 connected in series between the vdd rail and the ground rail. Inverter 12-22, which inverts input signal b and outputs the inverted signal /b, comprises a PMOS transistor 12 and an NMOS transistor 22 connected in series between the vdd rail and the ground rail. The analog switch 13-23, which opens and closes the electrical path between the output of the inverter 12-22 and the input of the inverter 16-26 in response to input a, comprises a PMOS transistor 13 and an NMOS transistor 23 connected in parallel. The tri-state inverter 14-25, which generates an output at the b logic level when input a is high and is in the high-impedance state when input a is low, comprises PMOS transistors 14, 15 and NMOS transistors 24, 25 connected in series between the vdd rail and the ground rail. The final-stage inverter 16-26, which generates the output signal y by inverting the output of the tri-state inverter 14-25 when input a is high and the output of the analog switch 13-23 when input a is low, comprises a PMOS transistor 16 and an NMOS transistor 26 connected in series between the vdd rail and the ground rail. When the inputs a, b are at matching logic levels (both high or both low), the output signal y goes to the low logic level. Otherwise (when the input logical levels do not match), the output signal y goes to the high logic level.
A conventional layout of the standard two-input NAND cell shown in FIG. 1 is shown in plan view in FIG. 6 and in a schematic sectional view in FIG. 7, which is taken through line Y1-Y2 in FIG. 6.
Referring to FIG. 6, the conventional standard two-input NAND cell is outlined by a rectangular cell boundary 30 with two opposite sides (the upper and lower sides in the drawing) disposed below the vdd rail 31 and ground rail 32. The vdd and ground rails 31, 32 are metal. A p-type (p+) semiconductor active area 33 and an n-type (n+) semiconductor active area 34 are disposed in the upper and lower parts, respectively, of the space between the rails 31, 32. An input terminal 35 for input signal a, an input terminal 36 for input signal b, and an output terminal 38 for the output signal y are aligned between the active areas 33, 34. PMOS transistor 1 occupies the left side of the p+ active area 33 and PMOS transistor 2 occupies the right side. NMOS transistor 3 occupies the right side of the n+ active area 34 and NMOS transistor 4 occupies the left side.
PMOS transistor 1 has a gate electrode 1g, a source region 1s, and a drain region 1d. The gate electrode 1g is a strip of polycrystalline silicon (polysilicon) extending generally vertically in the drawing across the p+ active area 33. The source region 1s and drain region 1d are highly doped p-type regions, referred to as p+ diffusion regions, disposed to the left and right, respectively, of the gate electrode 1g. The gate electrode 1g is connected through a contact plug to input terminal 35; the source region 1s is connected through a contact plug 31c to a short metal wire or stub 31a extending from the vdd rail 31 to a point above the source region is; the drain region 1d is connected through another contact plug and a metal wire 37 to the output terminal 38.
In the drawings, metal is indicated by hatching that slants toward the upper right, and polysilicon is indicated by hatching that slants toward the upper left.
PMOS transistor 2 has a gate electrode 2g, a source region 2s, and a drain region 2d. The gate electrode 2g is another strip of polysilicon extending generally vertically in the drawing across the p+ active area 33. The source region 2s and drain region 2d are p+ diffusion regions disposed to the right and left, respectively, of the gate electrode 2g. The gate electrode 2g is connected through a contact plug to input terminal 36; the source region 2s is connected through a contact plug 31c to another metal stub 31b extending from the vdd rail 31; the drain region 2d coincides with the drain region 1d of PMOS transistor 1 and is thus connected to the output terminal 38.
NMOS transistor 3 has a gate electrode 3g, a source region 3s, and a drain region 3d. The gate electrode 3g is a strip of polysilicon extending generally vertically in the drawing across the n+ active area 34. The source region 3s and drain region 3d are highly doped n-type regions (n+ diffusion regions) disposed to the left and right, respectively, of the gate electrode 3g, which is continuous with the gate electrode 2g of PMOS transistor 2. The drain region 3d is connected through a contact plug and the metal wire 37 to the output terminal 38.
NMOS transistor 4 has a gate electrode 4g, a source region 4s, and a drain region 4d. The gate electrode 4g is a strip of polysilicon extending generally vertically in the drawing across the n+ active area 34. The source region 4s and drain region 4d are n+ diffusion regions disposed to the left and right, respectively, of the gate electrode 4g, which is continuous with the gate electrode 1g of PMOS transistor 1. The source region 4s is connected through a contact plug 32c to a metal stub 32a extending from the ground rail 32. The drain region 4d coincides with the source region 3s of NMOS transistor 3.
As shown in FIG. 7, the standard two-input NAND cell is formed on an SOI wafer 40 comprising a silicon supporting substrate 41, a thick insulating film 42, and a thin silicon semiconductor film 43. The thick insulating film 42 is an oxide film, also referred to as a buried oxide or BOX film because it is buried between the silicon films 41, 43. The source and drain diffusion regions of the transistors 1, 2, 3, 4 are formed by implantation of impurity ions into the thin silicon semiconductor film 43, which is also referred to below as the SOI layer. Only source regions 1s and 4s are visible in FIG. 7. The diffusion regions are covered by an interlayer dielectric film 44 through which the source contact plugs 31c, 32c and other contact plugs mentioned above extend. The vdd rail 31, ground rail 32 and their stubs 31a, 31b, 32a, the input and output terminals 35, 36, 38, and the metal wire 37 are disposed in a lowermost metal wiring layer on the surface of the interlayer dielectric film 44.
FIG. 8 is a plan view of an exemplary conventional layout of the two-input XOR cell in FIG. 3. The layout includes a vdd rail 51 and a ground rail 52, which are metal strips disposed on two opposite sides (the upper and lower sides in the drawing) of the rectangular cell boundary 50. The lateral size of the cell boundary 50 in FIG. 8 is eight ‘grids’, that is, eight times the distance between adjacent lines in the grid of lines used to align the cell components when the cell is designed on, for example, a computer screen. The grid is indicated by vertical and horizontal lines in the drawing. The input and output terminals 55, 56, 58 are centered on grid intersections.
A p+ active area 53 and an n+ active area 54 are disposed in the upper and lower parts, respectively, of the space between the rails 51, 52. An input terminal 55 for input signal a, an input terminal 56 for input signal b, and an output terminal 58 for the output signal y are disposed in the space between the active areas 53 and 54. The PMOS transistors 11 to 16 and NMOS transistors 21 to 26 constituting the inverters 11-21, 12-22, analog switch 13-23, tri-state inverter 14-25, and final stage inverter 16-26 are formed in the active areas, PMOS transistors being formed in the p+ active area 53 and NMOS transistors in the n+ active area 54. The output terminals of the tri-state inverter 14-25 and analog switch 13-23 are connected through a metal wire 57 to the input terminal of the final stage inverter 16-26.
PMOS transistors 11 and 14 share a common source region that is connected through a contact plug to a metal stub 51a extending from the vdd rail 51. PMOS transistors 12 and 16 share a common source region that is connected through a contact plug to another metal stub 51b extending from the vdd rail 51. Similarly, NMOS transistors 21 and 24 share a common source region that is connected through a contact plug to a metal stub 52a extending from the ground rail, and NMOS transistors 22 and 26 share a common source region that is connected through a contact plug to another metal stub 52b extending from the ground rail 52. The vdd and ground rails 51, 52, their stubs 51a, 51b, 52a, 52b, the input and output terminals 55, 56, 58, the metal wire 57, and other metal interconnecting wires are all disposed in the same (e.g., lowermost) layer of metal wiring.
With the increasing density of semiconductor integrated circuits comes an increasing demand for reduced cell size. This demand can be met by reducing the sizes of the transistors constituting the standard cells, but only to a limited extent, because the driving capability of a transistor decreases when its size is reduced. An alternative method is to find a more compact layout, but in conventional standard cells described above, the compactness of the layout is limited by the following factors.
(i) The standard cell in FIG. 6 requires metal stubs 31a, 31b, 32a to connect the vdd rail 31 and ground rail 32 to the source regions of the PMOS transistors 1, 2 and NMOS transistor 4. These source regions 1s, 2s, 4s must be enlarged to make room for the contact plugs 31c, 32c below the stubs and to allow for alignment error.
(ii) The metal stubs 51b, 52b that connect the vdd and ground rails 51, 52 to the source regions of PMOS transistor 16 and NMOS transistor 26 in the final stage inverter 16-26 in FIG. 8 must fit into the space between the gate electrodes of these transistors 16, 26 and the metal wire 57 that connects those gate electrodes to the outputs of the tri-state inverter 14-25 and analog switch 13-23. Space must be left between this metal wire 57 and the metal stubs 51b, 52b, because they are disposed in the same metal wiring layer. As a result of these spacing requirements, the source areas 16s, 26s of transistors 16, 26 must have large lateral dimensions, making it hard to reduce the total width of the cell to less than the eight grids shown in FIG. 8.